More than likely due to nodal bias... or as I call it.. car pool syndrome. I imagine you have several parallel tasks all independent.. so does not matter which one is late, the successor starts late. Try this as an exercise... Correlate all your uncertainties 100%. Run the risk analysis again... If you have a much wider spread then question which tasks should be correlated.... For example should all Engineering electrical design be as same contractor... are the pipeline construction activities being carried out by same EPC.